Apparatus for dressing abrasive wheels

ABSTRACT

An automatically operated trimming device fed radially of a rotating abrasive wheel in response to a predetermined dimension change of the wheel to reduce its outside diameter. The wheel is movably mounted in a head and the head is moved forwardly by the amount of reduction in the wheel diameter. The trimming operation is initiated by movement of the wheel relative to the head toward a workpiece.

0 United States Patent 1151 3,634,977 Bunting 1 51 Jan. 18, 1972 [54] APPARATUS FOR DRESSING References Cited ABRASIVE WHEELS UNITED STATES PATENTS [721 Invent Elma", Cwemry, 3,155,086 11/1964 Ornehage 51/165 x gland 3,339,317 9/1967 Cleland.... ..51/5 x 73 Assignee; L Siegkr, Inc Santa Monica, Calif. 3,457,678 7/1969 Temple ..5 1/5 [22] Filed: 1969 Primary Examiner-Lester M. Swingle [2]] Appl- 799 220 Att0rneyWhittemore, Hulbert & Belknap [57] ABSTRACT 30 F'Al't'P"tDt I reign pp Ion "on y a a An automatically operated trimming device fed radially of 21 Feb. 16, 1968 Great Britain ..7,856/68 rotating abrasive wheel in response to a predetermined dimension change of the wheel to reduce its outside diameter. The [52] U.S.Cl ..5l/l65.87,5l/5 wheel is movably mounted in a head and the head is moved Cl --B24 4 forwardly by the amount of reduction in the wheel diameter. [58] Field of Search .L ..5 H165. 14, 165.15, 5 The trimming operation is initiated by movement of the wheel relative to the head toward a workpiece.

11 Claims, 12 Drawing Figures 32 31 /-4o 14 l 0 g PATENTED JANI a 1912 sum 1 or 8 vw mw vw i Cw Np o n We a Q 5 mm IHVENTOP THOMAS A. 6u-rmq BY I wmm, Mam M ATTORNEYS PATENTED JAN 1 8 1972 SHEET 2 OF 8 1 FIGS.

n u l i l l .J 108 106 INVEHTOF THOMAS A. BUHTINQ MMLJWp AT TORN EY PATENTED m 1 8 m2 SHEET 3 [1F 8 mmvfu IN VENTOR THOMAS mam/mm AT TORNEYS PATENIEU JAN 1 e 1912 SHEET 4 NB INVEN TOR THoMAs A-BvUNTIHG AT TORNEY PATENTEU JAN! 81972 SHEET 8 OF 8 FIGS.

INVENTOR THOMAS A. BUNTING W.M W

ATTORNEYS PATENTED JANE 81572 SHEET 7 [1F 8 hhm mum

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m jhzwwm ATTORNEYS APPARATUS FOR DRESSING ABRASIVE WHEELS This invention relates to an apparatus for, dressing an abrasive wheel to prepare that wheel for use in an abrasive finish machining operation on a workpiece. The invention has particular, but not exclusive, application to the dressing of the outside diameter of a gear hone which is used on a gear honing machine.

Where abrasive wheels are used for a finish machining operation such as in cylindrical grinding, or particularly in gear honing, it is necessary to periodically dress the outside diameter of the wheel to retrue the wheel after wear has occurred in use. In a gear honing operation where the workpiece gear is run in mesh with a toothed gear hone wheel at crossed axes, with the teeth of the gear hone wheel being conjugate to the desired tooth form on the workpiece gear; it is possible to control the honing condition so that as the gear hone wears during honing, the correct conjugate tooth form is continuously regenerated on the gear hone by the actual workpiece gear being honed. However as that regeneration progresses, there comes a time when the tops of the gear hone teeth contact the root diameter of the workpiece gear and it is then necessary to dress the outside diameter of the gear hone to top the gear hone teeth and so avoid such interference.

It is an object of the present invention to provide an apparatus for carrying out such a dressing operation automatically particularly on a gear honing machine or other similar type of machine. While particular reference is made in the remainder of this specification to gear honing machines, it will be appreciated that the apparatus is also applicable to the dressing of a grinding wheel such as may be used on a cylindrical grinding machine and it should, therefore, be understood that the scope of the invention is not limited to the gear honing machines which are described herein by way of illustration.

Conversely, it is to be understood that where reference is made to a wheel the term includes a gearlike abrasive honing tool. ,1

Accordingly the present invention provides on or for a gear honing or similar abrasive finishing machine comprising a workpiece support, a gear hone wheel or other abrasive finishing wheel carried for rotation on a head, means for detecting when said hone or wheel has worn by a predetermined amount, means for dressing said wheel in response to said detection means and means for advancing said head towards said workpiece support after such dressing to compensate therefore.

According to a further aspect of the present invention there is provided a means for dressing a gear hone wheel or other abrasive finishing wheel carried for rotation on a head of a gear honing machine or other similar abrasive finishing machine, comprising means for automatically measuring when said wheel has worn in use by a predetermined amount, means for said wheel in response to said measurement and for advancing said head to reposition said wheel to compensate for said dressing.

In order to promote a fuller understanding of the present invention, one embodiment will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. I is a general schematic side elevation of a gear honing machine incorporating the invention,

FIG. 2 is a cross-sectional view of the dressing head on the machine of FIG. 1,

FIG. 3 is a cross-sectional view on the line IIIIII of FIG. 2,

FIG. 4 is a part-sectional view on the line VIV of FIG. 2,

F IG. 5 is a cross-sectional view on the line V-V of FIG. 6 of the hone feed compensator on the machine of FIG. 1,

FIG. 6 is an elevational view of the compensating mechanism,

FIG. 7 is a schematic pneumatic circuit diagram of the hone dressing mechanism of the machine of FIG. 1,

FIG. 8 is a schematic electrical diagram view of the hone dressing mechanism of the machine of FIG. I,

FIG. 9 shows an alternative construction of the dressing head,

FIG. 10 shows a view on the line a of FIG. 9,

FIG. 11 shows an alternative construction of the hone feed compensator, and

FIG. 12 shows a view on the line Elk-E of FIG. 11.

Referring first to FIG. I, there is shown a schematic crosssectional view of a gear honing machine incorporating an embodiment of the invention. The machine comprises a base frame indicated at 10 having a gear honing head 11 mounted for movement thereon and a workpiece carrying table 12 also mounted thereon.

The table 12 is arranged to carry a gear workpiece indicated generally at 13 between running centers carried in head and tailstocks one of which is shown typically at 14.

The gear honing head 11 is mounted on a slide member 20 for sliding movement towards or away from the workpiece support 12 and workpiece I3 under the influence of a lead screw 2I. The arrangement is such that a gear hone wheel 22 which is carried on the gear honing head I] and arranged to be power driven by means of an electric motor 46 is brought into mesh with the workpiece gear 13 with the axis of the gear hone wheel and the workpiece gear arranged at an angle relative to one another to achieve a honing motion across the face width of the workpiece gear when the gear hone wheel is power driven. In order that the gear hone wheel may be traversed across the face width of the workpiece gear 13, the slide member 20 is mounted on the frame' 10 for sliding movement in a direction parallel to the axis of the gear workpiece on slideways indicated generally at 25, under the influence of a lead screw shown at 26.

In order that a workpiece gear may be honed to produce a slight crowning on the workpiece teeth, the table 12 is pivotally mounted on the machine frame I0 and is coupled by means of an adjustable cam mechanism 47 in known manner, to the slide member 20 so that the table 12 is rocked about a vertical axis, as seen in FIG. 1, during reciprocation'of the slide member 20 across the face widthof the workpiece gears.

As discussed above, the gear hone wheel 22 is brought into mesh with the workpiece gear 13 by means of the lead screw 21. However during honing the actual radial load between the hone 22 and the workpiece I3 is achieved, once the hone 22 has been brought into mesh with the workpiece 13 by the movement of the head 11 on slide member 20, by advancing the hone 22 relative to the head 11 by a small amount. The hone 22 is mounted for rotation on a quill (not shown) which is carried on a quill head 30 which is itself mounted for linear movement relative to the head 11 by means of a barrel 3! carried in a linear ball bearing assembly indicated at 32 in the head 11, and by upper sliding guideways (not shown) carried on a bracket 33 which also carries the hone drive motor 46. The adjustment of the angle of the axis of the hone 22 relative to the axis of the workpiece I3 is achieved by rotating the bracket 33, and with it the hone quill, relative to the head I] about the axis of barrel 31 to the desired angle in known manner.

The barrel 31, and consequently the hone 22 is advanced relative to the head II by means of a cam surface 40 carried on a loading member 41 which is mounted for vertical linear movement in the head 11 under the influence of weights 42. The cam surface 40 acts on the barrel 31 by means of a follower 43 pivotally mounted in the head 11 and having a cam following roller 44. The follower 43 engages on the end of the barrel 31 by means of a stiff leaf spring 45 which provides a resilient coupling to absorb honing shocks arising in operatron.

It can therefore be seen that when the cam surface 40 is allowed to proceed downwards under the influence of weights 42, the hone 22 will be advanced into honing engagement with the workpiece I3 with a predetermined force which is dependent upon the weights chosen at 42. Mechanism (not shown) is provided to lift the member 41 and thus the cam surface 40 and retract the hone in known manner and will not be described in further detail in this specification.

In FIG. I the hone 22 is shown schematically in its honing position in mesh with the workpiece gear I3, and it can be seen that as the honing teeth wear and are progressively regenerated the hone 22 will progress towards the axis of the workpiece under the influence of the weights 42. Further it can be seen that there will come a point when the tips of the hone teeth which are not worn during normal honing, contact the root diameter of the workpiece gear and that dressing of the outer diameter of the hone will be necessary if the regeneration is to continue unheeded.

A post 52 is provided on the quill head 30 and is arranged to extend through a slot in the bracket 33 adjacent a limit switch 53 (which in this embodiment is in the form of a proximity switch) fixed on the bracket 33. Consequently the limit switch 53 can be set to be operated by the post 52 when the quill head 30 and the hone 22 have progressed, relative to the bracket 33 and head 11, by a predetermined distance towards the workpiece axis and this distance is set to correspond to the travel necessary to bring the tips of the hone teeth into contact with root diameter of gear workpiece after the hone has worn. On operation of the limit switch 53, the hone 22 is dressed by a diamond dressing tool 50 carried on a dressing head 51 mounted on the quill head 30. The diamond tool 50 is arranged to top the teeth of the hone 22 and dress the outside diameter of the hone so that the tips of the teeth no longer contact the root diameter of the workpiece gear and honing can then continue with further regeneration of the hone teeth.

It can be seen that when the hone 22 is reduced in diameter by dressing as above, it is necessary that the operation of the limit switch 53 be reset so that it is operated again when the hone is again worn after further use. In order to achieve this, the head 11 is advanced towards the workpiece by an amount corresponding to the amount by which the hone 22 is reduced in radius during the dressing operation. As mentioned before, the head 11 is moved towards or away from the workpiece by means of a lead screw 21 and this advance is therefore achieved by rotating the lead screw 21 by a feed-compensating drive mechanism indicated generally at 60. The drive mechanism 60 is coupled to lead screw by way of gearing 61, a shaft 62 carried for rotation in the slide member 20 and a telescopic coupling shaft 63 which is provided with universal joints 64 to allow for the traverse movement of the slide member 20 along the slideways 25 relative to the feed mechanism 60 which is fixedly mounted on the frame 10, during honing ofa gear workpiece.

Thus it can be seen that as the head 11 is advanced after dressing, the limit switch 53 will again be set to detect when the hone has again worn and advanced relative to the workpiece sufficiently to require further dressing.

FIGS. 2, 3 and 4 show the dressing head 51 in more detail. The diamond dressing tool 50 is mounted in a screw-threaded holder 70 which is threaded into a bush 71 which is itself screw threaded into an end housing 72 which forms part of the housing 73 of the head 51. The bush 71 includes a gear 74 arranged in mesh with a rack 75 cut on an elongated piston 76 which is mounted for axial movement in a bore 77 in the housing 73, to form a double-acting pneumatic piston and cylinder device by which the bush 71 may be rotated in either direction.

The holder 70 is rigidly connected to the outer half ofa one way clutch indicated at 78, in this embodiment a Spragg clutch, the inner half of which is keyed to a locating shaft 79 carried in the housing 73. The inner halfof the clutch 78 is not permitted rotary movement relative to the locating shaft 79, but is allowed axial movement therealong. The locating shaft 79 is shouldered and threaded so that it may be locked in a bore formed in the housing 73 by means of a nut 80 when it has been adjusted to a given angular position in the housing 73 by means of the nut 81 which is locked on the end thereof.

The screw thread on the holder 70 and the mating thread in the bush 71 are chosen to be ofa smaller pitch than the screw thread by which the bush 71 engages with the housing 72. The one way clutch 78 is arranged to allow rotation of the holder 70 in the direction which would screw the holder 70 to the right as seen in FIG. 2 relative to the bush 71, but not in the opposite direction.

Thus it can be seen that on operation of the piston 76 to cause the bush 71 to rotate in the housing 72 so as to move the bush 71 to the right as seen in FIG. 2 which is the direction to engage and dress the hone, the holder 70 will be allowed to rotate with the bush 71 and consequently both will advance to the right to advance the diamond tool 50 for a dressing operation. In order to ensure that the holder 70 does rotate in this direction with the bush 71 without slippage, a nylon ring 82 is mounted in the bush 71 and is a tight but movable fit on the threaded outer diameter of the holder 70.

When the piston 76 is operated to rotate the bush 71 to retract the diamond tool, that is to say to move the bush 71 to left as seen in FIG. 2 it can be seen that the holder 70 is not permitted rotation in this direction by the clutch 78 and consequently the bush 71 will rotate relative to the holder 70 as well as housing 72 resulting in relative movement therebetween. Because of the difference in pitches of the screw threads, it will therefore be seen that although both the holder 70 and the bush 71 advance together, when the bush 71 retracts back to its original position the holder 70 will not be retracted by the full amount, resulting in the holder 70. and the diamond tool 50, returning to a new starting point which is slightly extended from the original one. The screw threads are chosen in pitch so that the difference in movement between the holder 70 and the bush 71 corresponds to the amount which it is desired to dress of the hone 22 at each dressing so that on each subsequent dressing operation an advance and return of the bush 71 by a predetermined amount will consistently remove a predetermined amount from the radius hone.

The scope of travel of the piston 76 in this embodiment is determined by a pin 83 which extends to the side of the housing 73 and is arranged to engage with a slot 84 in a removable piece 85, the length of the slot 84 being chosen to give the desired movement of the piston 76.

The locating rod 79 may be rotated to provide initial adjustment of the holder 70 relative to the bush 71 to bring the diamond 50 into an initial position, and then locked by the nut for subsequent further automatic advance of the diamond 50.

FIGS. 5 and 6 show the compensating mechanism 60 for the movement of the honing head 11 in more detail. The mechanism 60 comprises a housing which carried a stub shaft 101 for rotation therein, arranged in driving engagement at one end with one of the universal joints 64 of the telescopic shaft 63 and provided with a squared portion at the other end to which a handwheel may be attached for manual movement of the head 11. A gear 103 is carried for free rotation on the shaft 101 by means ofa bush 104 and may be drivably coupled to the shaft 101 by means of an electromagnetic clutch 105 to which electrical power may be carried in known manner by means of slip ring and brush gear indicated at 106.

The gear 103 is arranged in mesh with a rack 108 which is slidably mounted in the housing 100 and may be moved by a double-acting pneumatic piston and cylinder device 109. Thus operation of the device 109 with simultaneous engagement of the clutch 105 may be disengaged during retraction of the device 109 to the starting position again ready for further rotation of the lead screw 21. The extent of travel of the rack 108 may be controlled by an adjustable stop 110 which is screw threaded into the housing 100. The adjustment of the stop 110 is arranged so that the lead screw 21 advances the head 11 towards the workpiece 13 by an amount corresponding to that dressed off the radius of the hone 21 by the diamond tool 50.

FIG. 7 shows, in schematic form, the pneumatic circuit diagram for controlling the dressing of the hone as described above. Where parts shown in FIG. 7 correspond to those previously described, they are given the same reference numerals.

As can be seen, the operation of the double-acting pneumatic cylinder formed by piston 76 is controlled by a solenoidoperated valve which, on operation of solenoid 121, switches air pressure from a supply 122 to move the piston 76 to rotate the gear 74 so as to extend the diamond tool 50 to dress the hone, or on operation of solenoid 123 to move the piston 76 to retract the diamond tool 50. Speed control valves 124 are provided to operate in known manner on each side of the piston 76 to restrict the exhaust flow of air therefrom.

Similarly, the pneumatic piston and cylinder device 109 is controlled by a solenoid-operated control valve 125 which is operable by a solenoid 126, to cause device 109 to rotate the lead screw 21 to feed the hone further towards the workpiece gear, or by the solenoid 127 to cause device 109 to return the rack 108 to its initial position. Again speed control valves 124 are provided in known manner.

FIG. 8 shows, in schematic form, the electrical circuit for controlling the operation of the hone dressing automatically.

The limit or proximity switch 53, when it is operated closes two pairs of contacts 53A and 53B. Contacts 53B cause an indicator lamp 200, positioned on the machine control panel, to indicate to the operator of the machine that the hone dressing sequence has been indicated. Contacts 53 initiate a timer relay 201, which when closed after a short delay energizes one-half 202 of a latched pair of relays by way of contacts 201A.

The timer relay 201 is provided and arranged so that it only closes after the proximity switch has energized it for a predetermined time. This is to prevent the hone dressing sequence being initiated by a transient signal from the proximity switch 53 caused by an eccentric workpiece gear or even an eccentric hone. The timer and relay 201 only closes after a steady signal from the proximity switch 53 spread over at least a revolution of the hone or workpiece.

When the relay half 202 is energized it in turn energizes relay 203 which in turn energizes the solenoid 126 to advance the hone compensator mechanism and inhibits the operation of the solenoid 125 which would retract the hone compensator mechanism, by way of contacts 203A and 203B respectively; Additional contacts 203C and 203D on relay 203 energize the clutch 105 by way of a transformer 204 and rectifier system 205 so that the compensator mechanism is engaged in the advance direction. Relay 202 also energizes solenoid 121 by way of contacts 2023 to advance the hone dressing tool 50 to its dressing position.

At the beginning of the next cycle of the honing machine contacts 206 on a relay in the main machine control circuitry (not shown) are closed to energize the second half 207 latched to relay 202 with the result that relay 202 is cancelled. Following from this relay 203 is deenergized resulting in the clutch 105 disengaging and the compensator mechanism is retracted leaving the head 11 advanced an amount appropriate to that dressed off the hone.

At the next cycle ofthe machine also, when the hone quill head 30 retracts, the hone dressing tool 50 is retracted by solenoid 123 being energized from line 208 which is energized by the main machine control circuitry at that time in the machine cycle. The line 208 is arranged to be energized, from the main machine control circuitry, only when the machine is set in an automatic cycle as opposed when various machine elements are moved under individual manual control.

FIGS. 9 and 10 show an alternative construction to the dressing head shown in FIGS. 2, 3 and 4. Where parts in FIGS. 9 and 10 correspond to those previously described they are given the same reference numbers only increased by 300.

In this alternative arrangement the one-way clutch 378 is mounted with its inner half mounted on a shaft 386 which is keyed to the holder 370 to rotate therewith, but to allow relative axial movement. The outer half of the clutch 378 is mounted on a housing 387 which is rotatably carried in the housing 373. The end housing 387 may be clamped in a given rotational position by means of a setscrew provided in a cover cap 351 which is fixed to the housing 373 to axially retain the end housing 387. The shaft 386 is provided with a squared end whereby axial adjustment may be made to the setting of the dressing tool 50 and this may subsequently be held in adjustment by the setscrew.

Further in this alternative arrangement no provision is made for adjusting the scope of movement of the piston 376, the

scope being determined by the distance between the end plates 390 in the housing 373 and the length of the piston 376.

FIGS. 11 and 12 show an alternative construction of the compensating device shown in FIGS. 6 and 7. Where parts in FIGS. 11 and 12 correspond to those previously described they are given the same reference numbers only increased by 300.

In this alternative arrangement the piston and cylinder device 109 is replaced by a piston 41] carried in a bore 412 in the housing 400 and having a rack 413 cut on one side to engage with the gear 403. The piston 411 is formed with a larger diameter at one end so as to give a larger operative thrust to advance the hone head 11 than the thrust needed to retract the device with the clutch 405 disengaged. The slipring mechanism 406 for the clutch 405 is also moved to the side of the housing 400 so as not to interfere with the piston 411.

The scope of the movement of the piston 411, and thus the scope of the compensation given by this alternative arrangement is controlled by the distance between the end plates 414 on the bore 412 and the length of the piston 41].

Thus, it can be seen that there is provided by embodiment of the invention a dressing mechanism for a gear hone on a gear honing machine which will automatically dress the hone to top the teeth thereon and reduce the overall diameter of the hone when this is detected as being necessary for the continued correct operation of the hone. It will be appreciated further that this embodiment of the invention could be readily modified to dress a grinding wheel on a cylindrical grinding machine when that has worn in use and to reset the position of the grinding wheel spindle to compensate for that dressing.

Iclaim:

1. An abrasive finishing machine tool comprising a base, a workpiece support mounted on said base, a wheel head movably mounted on said base, and an abrasive finishing wheel carried for rotation by said wheel head, means for detecting when said wheel has worn in use'by a predetermined amount, means for dressing the outside diameter of said wheel by a chosen reduction in radius in response to said detecting means, and means for advancing said wheel head toward a workpiece carried on said workpiece support to reposition said wheel to compensate for such dressing, said wheel being carried in a quill head which is movably mounted in said wheel head so as to be movable between a forward position in engagement with a workpiece and a retracted position clear of a workpiece, said detecting means comprising a limit switch arranged to detect forward movement of said quill head relative to said wheel head towards a workpiece to predetermined position relative to said wheel head.

2. A machine tool as claimed in claim 1 wherein said compensating means is arranged to move said wheel towards a workpiece by the chosen reduction dressed off the outer diameter of said wheel.

3. A machine tool as claimed in claim 1, wherein said dressing means comprises a dressing tool mounted adjacent said wheel and arranged to be advanced into dressing engagement by a first distance and subsequently retracted from said wheel by a second distance on each such dressing, said first less said second distance being equal to the chosen reduction to be dressed off said wheel.

4. A machine tool as claimed in claim 3, wherein said dressing tool is carried in a holder, said holder is rotatably carried by means of a first screw threading of a first axial pitch in a bush, said bush is rotatably carried by means of a second screw threading of a second axial pitch in a housing which is mounted on said quill head, means is provided to rotate said bush by a fixed amount in either direction to advance and then retract said tool to dress and then clear said wheel and means is provided to allow said holder to rotate with said bush in the advance directionand to prevent said holder from rotating in said retract direction, said first axial pitch being greater than said second axial pitch by an amount directly proportional to said chosen reduction of said wheel.

5. A machine tool as claimed in claim 4, wherein said means for rotating said bush comprises a gear provided on said bush in engagement with a rack slidably mounted in said housing and operable over a predetermined stroke by a double-acting fluid pressure piston and cylinder device.

6. A machine tool as claimed in claim 4, wherein said means for allowing said holder to rotate with said bush in the advance direction and to prevent said holder from rotating in said retract direction comprises a one-way clutch operatively arranged between said holder and said housing.

7. A machine tool as claimed in claim 2, wherein said wheel head is movable on said base by means of a lead screw and said lead screw is arranged to be rotated by said compensating means which comprises a compensator housing, a gear rotatably mounted in said compensator housing arranged to drive said lead screw by way of a clutch, a rack slidably mounted in said compensator housing and movable by a double-acting fluid pressure piston and cylinder device, said clutch being engaged when said rack is moved to advance said wheel head and disengaged during the return stroke of said rack.

8. A machine tool as claimed in claim 2 wherein said limit switch is a proximity switch arranged to sense the approach of a probe arranged to move with said wheel.

9. A machine tool as claimed in claim 2 wherein said limit switch is arranged to energize a timer relay, said timer relay being adapted to close electrical contacts thereon to initiate the electrical control of such wheel dressing after a time interval corresponding to at least one revolution of said wheel in use.

10. A machine tool as defined in claim 1 in which said wheel is carried by said hone head for relative movement with respect thereto, resilient means acting between said hone head and wheel effective to urge said wheel toward a workpiece with a substantially constant force in which the means for detecting when the wheel has worn in use by a predetennined amount comprises means for sensing movement of said wheel relative to said hone head.

11. A gear honing machine comprising a base, a rotary work support on said base, a hone slide movable on said base toward and away from said work support, a rotary hone support mounted on said hone slide for movement relative thereto toward and away from said work support, resilient means acting between said hone support and hone slide urging said hone support forwardly to a position determined by engagement between a work gear on said work support and a hone on said hone support, sensing means responsive to predetermined forward movement of said hone support on said hone slide to trim the hone to reduce its outside diameter by a predetermined amount, and means for thereafter advancing said hone slide by an equivalent amount to reset the sensing means. 

1. An abrasive finishing machine tool comprising a base, a workpiece support mounted on said base, a wheel head movably mounted on said base, and an abrasive finishing wheel carried for rotation by said wheel head, means for detecting when said wheel has worN in use by a predetermined amount, means for dressing the outside diameter of said wheel by a chosen reduction in radius in response to said detecting means, and means for advancing said wheel head toward a workpiece carried on said workpiece support to reposition said wheel to compensate for such dressing, said wheel being carried in a quill head which is movably mounted in said wheel head so as to be movable between a forward position in engagement with a workpiece and a retracted position clear of a workpiece, said detecting means comprising a limit switch arranged to detect forward movement of said quill head relative to said wheel head towards a workpiece to predetermined position relative to said wheel head.
 2. A machine tool as claimed in claim 1 wherein said compensating means is arranged to move said wheel towards a workpiece by the chosen reduction dressed off the outer diameter of said wheel.
 3. A machine tool as claimed in claim 1, wherein said dressing means comprises a dressing tool mounted adjacent said wheel and arranged to be advanced into dressing engagement by a first distance and subsequently retracted from said wheel by a second distance on each such dressing, said first less said second distance being equal to the chosen reduction to be dressed off said wheel.
 4. A machine tool as claimed in claim 3, wherein said dressing tool is carried in a holder, said holder is rotatably carried by means of a first screw threading of a first axial pitch in a bush, said bush is rotatably carried by means of a second screw threading of a second axial pitch in a housing which is mounted on said quill head, means is provided to rotate said bush by a fixed amount in either direction to advance and then retract said tool to dress and then clear said wheel and means is provided to allow said holder to rotate with said bush in the advance direction and to prevent said holder from rotating in said retract direction, said first axial pitch being greater than said second axial pitch by an amount directly proportional to said chosen reduction of said wheel.
 5. A machine tool as claimed in claim 4, wherein said means for rotating said bush comprises a gear provided on said bush in engagement with a rack slidably mounted in said housing and operable over a predetermined stroke by a double-acting fluid pressure piston and cylinder device.
 6. A machine tool as claimed in claim 4, wherein said means for allowing said holder to rotate with said bush in the advance direction and to prevent said holder from rotating in said retract direction comprises a one-way clutch operatively arranged between said holder and said housing.
 7. A machine tool as claimed in claim 2, wherein said wheel head is movable on said base by means of a lead screw and said lead screw is arranged to be rotated by said compensating means which comprises a compensator housing, a gear rotatably mounted in said compensator housing arranged to drive said lead screw by way of a clutch, a rack slidably mounted in said compensator housing and movable by a double-acting fluid pressure piston and cylinder device, said clutch being engaged when said rack is moved to advance said wheel head and disengaged during the return stroke of said rack.
 8. A machine tool as claimed in claim 2 wherein said limit switch is a proximity switch arranged to sense the approach of a probe arranged to move with said wheel.
 9. A machine tool as claimed in claim 2 wherein said limit switch is arranged to energize a timer relay, said timer relay being adapted to close electrical contacts thereon to initiate the electrical control of such wheel dressing after a time interval corresponding to at least one revolution of said wheel in use.
 10. A machine tool as defined in claim 1 in which said wheel is carried by said hone head for relative movement with respect thereto, resilient means acting between said hone head and wheel effective to urge said wheel toward a workpiece with a substantially constant force in whicH the means for detecting when the wheel has worn in use by a predetermined amount comprises means for sensing movement of said wheel relative to said hone head.
 11. A gear honing machine comprising a base, a rotary work support on said base, a hone slide movable on said base toward and away from said work support, a rotary hone support mounted on said hone slide for movement relative thereto toward and away from said work support, resilient means acting between said hone support and hone slide urging said hone support forwardly to a position determined by engagement between a work gear on said work support and a hone on said hone support, sensing means responsive to predetermined forward movement of said hone support on said hone slide to trim the hone to reduce its outside diameter by a predetermined amount, and means for thereafter advancing said hone slide by an equivalent amount to reset the sensing means. 